The present invention relates to an optical module having a substrate on which a light emitting device and a photodetector are mounted.
For example, in an optical transmission system with the function of bidirectional transmission, a bidirectional optical transmission module with a light emitting device and a photodetector incorporated inside is used. According to recent developments in computer hardware and networks, a reduction in the size and cost of bidirectional optical transmission modules has been demanded since the module has come to be installed domestically.
A method for reduction in size and cost, wherein a part is incorporated into a package and connected to optical fibers for transmission and/or waveguide parts, is suitable in terms of the manufacturing and assembling processes. The part comprises a substrate with conductor inter connections laid as necessary, wherein a light emitting device and a photodetector are mounted on the substrate.
In this case, an Si substrate is preferable as the substrate to be provided with a light emitting device and a photodetector. It is possible to acquire a Si material with stable quality at a low price, and the Si material has excellent in workability, so that formation of alignment marks for positioning a light emitting device and a photodetector when mounting them, and formation of V-grooves for positioning optical fibers are easy.
In the present specification, an Si substrate to be used for such a purpose, or an Si substrate which has a light emitting device and a photodetector and conductor inter connection provided on it with alignment marks and V-grooves for positioning formed as necessary, is called an SOB (Silicon Optical Bench).
FIG. 1 shows a conception diagram of the SOB. In FIG. 1, the reference numeral 105 denotes a substrate with a thickness of, for example, 1 mm, on which conductor (inter connection) patterns 106 are formed via insulation layer 107 made from SiO2 or the like.
Light emitting device 104 and photodetector 101 are mounted on the substrate 105, and connected to the conductor patterns 106 on the substrate 105 by soldering and bonding wires 102. Furthermore, the reference-numeral 108 denotes a leader inter connection conductor, and the reference numeral 109 denotes a dielectric.
FIG. 2 is a drawing showing the entire construction of the SOB. In this figure, in addition to the light emitting device (LD) 104 and the photodetector (PO) 101, a photodetector (MPD) for detecting and controlling the output of the light emitting device 104 is disposed at the rear stage of the light emitting device 104. On the substrate 105, an alignment mark is formed for positioning and mounting the light emitting device 104 and photodetector 101, along with V-grooves to be used for positioning and connecting the SOB to external optical fibers (ferrules) and waveguides.
The SOB thus constructed is packaged together with a connection structure for connection with external optical fibers, for preamplifiers, external connection terminals, or optical ferrules, for example, and is thus completed as an optical module (see FIG. 3).
For realizing reduction in the size of the optical module, it is preferable that the size of the substrate on which the light emitting device and the photodetector are mounted is reduced. For example, the current standard pitch of an optical fiber ribbon is 250 xcexcm, and if connection with the optical fiber ribbon is set as a precondition, the application of the module in which the light emitting device and the photodetector are arranged at a 250 xcexcm pitch is most ideal.
Furthermore, the light emitting device is generally driven by a current of 10 mA or more, however, a photocurrent generated from the photodetector is smaller by several digits than the drive current, which is normally in the order of xcexcA at most. Particularly, in optical transmission, since it is demanded that the relay stations be reduced and transmission be made with more receiving stations, it is important to increase the photosensitivity of the photodetector. For example, in a certain standard, a minimum receiving sensitivity of xe2x88x9230 dBm (0.001 mW) or less is required for an application of several-hundred Mb/s, and also, some cases require detection of a photocurrent of xcexcA or less.
In examination of reducing the size of a module and photodetector""s detection of a small photocurrent, an important factor in the development of an application, in which the light emitting device and photodetector are mounted on an Si substrate, is to solve the problem of electric crosstalk generated by signals of the light emitting device side influencing the photodetector side. The present invention provides an optical module wherein the size thereof is reduced and the problem of crosstalk is solved.
The present invention provides an optical module having the following construction. That is, the optical module of the invention has a substrate which has a photodetecor and a light emitting device mounted on it and is provided with a conductor wall between the photodetector and the light emitting device. The conductor wall is fitted and fixed, a necessary, into a groove portion formed in the substrate.
In a more preferable embodiment, the conductor wall is provided with a top plate positioned above one or both of the photodetector and the light emitting device. The conductor wall may be formed so as to have a T-shaped section or an L-shaped section. As another embodiment, the conductor wall may be composed of a first portion formed to have an L-shaped section as mentioned above and a second portion forming a top plate which has a bar-shaped section and is provided independently from said first portion.
In one embodiment, the conductor wall is connected to at least one of a ground provided in a drive circuit of the light emitting device and a ground provided in an amplifier circuit for electric signals outputted from the photodetector. In this case, the conductor wall may be formed of a plurality of conductors which are independent and separate from each other, and the grounds of each conductor may be separately provided.
In another embodiment, the conductor wall of the invention may be formed by forming a conductive film on the surface of a wall member made of a resin mold.
Electric crosstalk is considered to occur between the photodetectors and light emitting device themselves, the substrate, inter connection patterns, and bonding wires or the like its mechanism is complicated. However, as in the abovementioned construction, electric crosstalk from the light emitting device can be effectively suppressed by forming the conductor wall between the photodetector and the light emitting device.
The present invention solves the problem of electric crosstalk from signals of the light emitting device side, which harmfully influences the photodetector side, by a simple construction in which a conductor wall is formed between the photodetector and the light emitting device. The optical module of the invention is excellent for practical use since an effect of remarkably reduced crosstalk can be obtained by a means without disadvantages such as significant increases in the material cost, the number of processes, the substrate""s size, and substantial deterioration in the characteristics of the optical module.
The invention is to be applied to an optical module, and is useful for an optical module in which xe2x88x9220 dBm is required as the minimum photosensitivity of said photodetector, a normal drive current of said light emitting device is 10 mA or more. The photodetector and the light emitting device which are in the most proximity to each other are disposed at a pitch of 1 mm or less (pitch is the distance between the centers of the light emitting device and photodetector).
In short, the present invention aims to effectively suppress crosstalk from the light emitting device to the photodetector by employing a means which can be easily employed industrially without complicated processing, an increase in the material cost, and harmful influences on the characteristics of the optical module. The invention is useful in various industrial fields for manufacturing and using optical modules having substrates on which light emitting devices and photodetectors are mounted.
Particularly, in the invention, the conductor wall is provided with a top plate to cover the photodetector, whereby the electromagnetic shield effect can be increased. A construction is employed in which the conductor wall is further provided with a top plate to cover the light emitting device, whereby the electromagnetic effect can be further increased.
Moreover, the conductor wall having the top plates is constructed so as to have a T-shaped section or an L-shaped section, or is composed of a first portion formed to have an L-shaped section and the second portion formed to have a bar-shaped section and provided independently from said first portion. The provision of the conductor thus constructed allows the optical module having the abovementioned excellent effect to be easily formed.
Furthermore, in the invention, the optical module is an optical part having at least a photodetector and a light emitting device on one side surface, and substantially consists of an Si substrate as a main body which has the photodetector and the light emitting device mounted on it. In addition, in the invention, the optical module is an optical part including a substrate as one component which has the photodetector and the light emitting device mounted on it, that is, an optical part including, as other components, an optical connection mechanism such as an MT-RJ connector and a ferrule for connecting the photodetector and light emitting device to optical fibers or waveguides, and/or inter connection and terminals for detecting and amplifying the photocurrent of the photodetector and applying and detecting the drive voltage of the light emitting device, and/or a casing, and/or optical fibers.